The NAS displayed error E13 and E26 and stopped working.
The customer performed a firmware update. The NAS then displayed error code E30 (drive malfunctioning).

Diagnosis:

RAID rotation analysis confirmed a RAID 6 configuration.

One drive did not belong to the RAID and a second drive was stale. Effectively this was a 4-drive RAID 6 without two drives. A RAID 6 without two drives is recoverable
if the algorithm to generate the Q-parity data is known. The algorithm used by this NAS was not published by the manufacturer.

Our analysis also showed that the RAID hosted an XFS volume. The
primary super block
of the volume was lost.

Solution:

We were able to determine that the Q-parity data was based on the widely-used Reed-Solomon encoding scheme.

We manually recreated the missing XFS metadata.

We then ran File Scavenger® and recovered the data. File Scavenger® version 4.3 or later supports RAID 6 that uses the Reed-Solomon encoding scheme.